Specify source level 1.6 for GWT.
Specify source level 1.6 for GWT.

This fixes failures in guava-gwt when building/testing under JDK8.
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82526985

easyfmxu/guava,mway08/guava,mosoft521/guava,ignaciotcrespo/guava,codershamo/guava,dpursehouse/guava,XiWenRen/guava,GabrielNicolasAvellaneda/guava,thinker-fang/guava,dmi3aleks/guava,r4-keisuke/guava,disc99/guava,flowbywind/guava,witekcc/guava,pwz3n0/guava,1yvT0s/guava,jedyang/guava,GitHub4Lgfei/guava,danielnorberg/guava-libraries,elijah513/guava,paplorinc/guava,jsnchen/guava,lisb/guava,renchunxiao/guava,montycheese/guava,tli2/guava,codershamo/guava,XiWenRen/guava,liyazhou/guava,Yijtx/guava,abel-von/guava,KengoTODA/guava,Kevin2030/guava,jankill/guava,SaintBacchus/guava,paplorinc/guava,jackyglony/guava,dushmis/guava,rob3ns/guava,kaoudis/guava,cgdecker/guava,tobecrazy/guava,monokurobo/guava,mgalushka/guava,BollyCheng/guava,VikingDen/guava,typetools/guava,eidehua/guava,m3n78am/guava,lgscofield/guava,rgoldberg/guava,0359xiaodong/guava,qingsong-xu/guava,mgalushka/guava,Yijtx/guava,njucslqq/guava,kingland/guava,weihungliu/guava,kingland/guava,norru/guava,sunbeansoft/guava,taoguan/guava,AnselQiao/guava,gmaes/guava,easyfmxu/guava,BollyCheng/guava,hannespernpeintner/guava,liyazhou/guava,mohanaraosv/guava,jiteshmohan/guava,juneJuly/guava,tobecrazy/guava,disc99/guava,kaoudis/guava,1yvT0s/guava,uschindler/guava,DavesMan/guava,jsnchen/guava,yanyongshan/guava,Akshay77/guava,Xaerxess/guava,RoliMG/guava,aiyanbo/guava,seanli310/guava,dpursehouse/guava,leogong/guava,jakubmalek/guava,leogong/guava,leesir/guava,allenprogram/guava,licheng-xd/guava,jiteshmohan/guava,ben-manes/guava,rcpoison/guava,yuan232007/guava,rcpoison/guava,xasx/guava,google/guava,huangsihuan/guava,typetools/guava,njucslqq/guava,fengshao0907/guava,norru/guava,montycheese/guava,sebadiaz/guava,lijunhuayc/guava,leesir/guava,elijah513/guava,5A68656E67/guava,fengshao0907/guava,ceosilvajr/guava,Kevin2030/guava,cklsoft/guava,sarvex/guava,jedyang/guava,binhvu7/guava,juneJuly/guava,5A68656E67/guava,jayhetee/guava,yigubigu/guava,AnselQiao/guava,manolama/guava,qingsong-xu/guava,anigeorge/guava,DavesMan/guava,thinker-fang/guava,levenhdu/guava,tunzao/guava,aditya-chaturvedi/guava,renchunxiao/guava,anigeorge/guava,huangsihuan/guava,mosoft521/guava,Akshay77/guava,monokurobo/guava,DucQuang1/guava,cklsoft/guava,GabrielNicolasAvellaneda/guava,tli2/guava,yuan232007/guava,xasx/guava,VikingDen/guava,flowbywind/guava,levenhdu/guava,jakubmalek/guava,marstianna/guava,rgoldberg/guava,witekcc/guava,google/guava,Xaerxess/guava,licheng-xd/guava,allenprogram/guava,mway08/guava,abel-von/guava,SyllaJay/guava,hannespernpeintner/guava,DucQuang1/guava,ningg/guava,weihungliu/guava,jayhetee/guava,jankill/guava,google/guava,ChengLong/guava,yanyongshan/guava,EdwardLee03/guava,mosoft521/guava,manolama/guava,sunbeansoft/guava,aiyanbo/guava,jackyglony/guava,HarveyTvT/guava,pwz3n0/guava,lijunhuayc/guava,m3n78am/guava,lgscofield/guava,mohanaraosv/guava,eidehua/guava,dushmis/guava,SyllaJay/guava,yigubigu/guava,berndhopp/guava,GitHub4Lgfei/guava,EdwardLee03/guava,scr/guava,aditya-chaturvedi/guava,KengoTODA/guava,lisb/guava,0359xiaodong/guava,ChengLong/guava,SaintBacchus/guava,Ariloum/guava,marstianna/guava,HarveyTvT/guava,RoliMG/guava,ignaciotcrespo/guava,Ariloum/guava,r4-keisuke/guava,sebadiaz/guava,scr/guava,berndhopp/guava,taoguan/guava,rob3ns/guava,gmaes/guava,ningg/guava,seanli310/guava,tunzao/guava