fix build script
fix build script

Summary: travis-ci (12.04) needs manually installed double-conversion

Test Plan: test

Reviewed By: @pavlo-fb

Differential Revision: D1922642
tempbottle/mcrouter,seem-sky/mcrouter,nvaller/mcrouter,reddit/mcrouter,nvaller/mcrouter,nvaller/mcrouter,is00hcw/mcrouter,zhlong73/mcrouter,yqzhang/mcrouter,facebook/mcrouter,tempbottle/mcrouter,easyfmxu/mcrouter,reddit/mcrouter,seem-sky/mcrouter,leitao/mcrouter,tempbottle/mcrouter,reddit/mcrouter,glensc/mcrouter,nvaller/mcrouter,zhlong73/mcrouter,is00hcw/mcrouter,yqzhang/mcrouter,facebook/mcrouter,easyfmxu/mcrouter,evertrue/mcrouter,leitao/mcrouter,glensc/mcrouter,glensc/mcrouter,evertrue/mcrouter,zhlong73/mcrouter,seem-sky/mcrouter,easyfmxu/mcrouter,reddit/mcrouter,apinski-cavium/mcrouter,seem-sky/mcrouter,evertrue/mcrouter,facebook/mcrouter,facebook/mcrouter,yqzhang/mcrouter,zhlong73/mcrouter,tempbottle/mcrouter,synecdoche/mcrouter,glensc/mcrouter,synecdoche/mcrouter,apinski-cavium/mcrouter,apinski-cavium/mcrouter,synecdoche/mcrouter,yqzhang/mcrouter,evertrue/mcrouter,synecdoche/mcrouter,is00hcw/mcrouter,easyfmxu/mcrouter,leitao/mcrouter,apinski-cavium/mcrouter,leitao/mcrouter,is00hcw/mcrouter