Two reasons here:
1) Ubuntu Precise is reaching EOL, though Travis CI may take care of it.
we still can move to Trusty earlier as I didn't see any dependency of
Precise here.
2) GCC 4.8 is built-in in Travis CI's Ubuntu Trusty environment, which
means we don't need to spend time on adding ppa repository, apt update
and apt install, could save time on the CI.
Ref:
https://docs.travis-ci.com/user/trusty-ci-environment/#Compilers-%26-Build-toolchain
