Revert "Try to update to GCC 6 on Travis."
Revert "Try to update to GCC 6 on Travis."

This reverts commit 55e1dd6203764bf09877a0134cf557860a0e5a53.

mono/CppSharp,ktopouzi/CppSharp,ktopouzi/CppSharp,ktopouzi/CppSharp,mono/CppSharp,u255436/CppSharp,ddobrev/CppSharp,mono/CppSharp,inordertotest/CppSharp,zillemarco/CppSharp,mono/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,mono/CppSharp,inordertotest/CppSharp,zillemarco/CppSharp,zillemarco/CppSharp,u255436/CppSharp,inordertotest/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,u255436/CppSharp,u255436/CppSharp,inordertotest/CppSharp,mohtamohit/CppSharp,mohtamohit/CppSharp,inordertotest/CppSharp,ktopouzi/CppSharp,mono/CppSharp,u255436/CppSharp,mohtamohit/CppSharp,mohtamohit/CppSharp,mohtamohit/CppSharp,ktopouzi/CppSharp,zillemarco/CppSharp,zillemarco/CppSharp