travis: Remove redundant travis tests (#157)
travis: Remove redundant travis tests (#157)

* Avoid doing pretty check on both Linux and OSX platforms.
* Avoid testing the compilation of `CC2538` on both OS X and Linux.

This will help reduce the time it takes for a travis validation.
jjlee9/openthread,srickardti/openthread,lanyuwen/openthread,pvanhorn/openthread,jjlee3/openthread,georgecpr/openthread,vaas-krish/openthread,erja-gp/openthread,bukepo/openthread,srickardti/openthread,mszczodrak/openthread,Zolertia/openthread,chshu/openthread,bukepo/openthread,LeZhang2016/openthread,georgecpr/openthread,openthread/openthread,erja-gp/openthread,lanyuwen/openthread,LeZhang2016/openthread,pvanhorn/openthread,GiedriusM/openthread,nibanks/openthread,vaas-krish/openthread,mszczodrak/openthread,aeliot/openthread,fbsder/openthread,openthread/openthread,aeliot/openthread,jjlee9/openthread,nibanks/openthread,xiaom-GitHub/openthread,mszczodrak/openthread,mszczodrak/openthread,jwhui/openthread,JakubBrachTieto/openthread,erja-gp/openthread,srickardti/openthread,xiaom-GitHub/openthread,vaas-krish/openthread,xiaom-GitHub/openthread,pvanhorn/openthread,fbsder/openthread,mszczodrak/openthread,georgecpr/openthread,jjlee9/openthread,xiaom-GitHub/openthread,pvanhorn/openthread,xiaom-GitHub/openthread,JakubBrachTieto/openthread,turon/openthread,jjlee9/openthread,jjlee3/openthread,JakubBrachTieto/openthread,erja-gp/openthread,GiedriusM/openthread,jwhui/openthread,nibanks/openthread,fbsder/openthread,abtink/openthread,lanyuwen/openthread,LeZhang2016/openthread,jjlee3/openthread,vaas-krish/openthread,gandreello/openthread,Zolertia/openthread,lanyuwen/openthread,aeliot/openthread,abtink/openthread,jjlee9/openthread,bukepo/openthread,nibanks/openthread,gandreello/openthread,aeliot/openthread,GiedriusM/openthread,georgecpr/openthread,lanyuwen/openthread,turon/openthread,fbsder/openthread,JakubBrachTieto/openthread,chshu/openthread,aeliot/openthread,pvanhorn/openthread,mszczodrak/openthread,georgecpr/openthread,librasungirl/openthread,chshu/openthread,georgecpr/openthread,GiedriusM/openthread,Zolertia/openthread,gandreello/openthread,LeZhang2016/openthread,nibanks/openthread,pvanhorn/openthread,fbsder/openthread,turon/openthread,fbsder/openthread,jjlee3/openthread,openthread/openthread,jwhui/openthread,chshu/openthread,jjlee9/openthread,chshu/openthread,LeZhang2016/openthread,turon/openthread,jwhui/openthread,aeliot/openthread,librasungirl/openthread,nibanks/openthread,bukepo/openthread,gandreello/openthread,gandreello/openthread,jjlee3/openthread,LeZhang2016/openthread,gandreello/openthread,lanyuwen/openthread,librasungirl/openthread,librasungirl/openthread,Zolertia/openthread,JakubBrachTieto/openthread,GiedriusM/openthread,JakubBrachTieto/openthread,abtink/openthread,vaas-krish/openthread,openthread/openthread,erja-gp/openthread,srickardti/openthread,erja-gp/openthread,Zolertia/openthread,xiaom-GitHub/openthread,abtink/openthread,chshu/openthread,vaas-krish/openthread