Move to Ubuntu Trusty on Travis CI and remove apt add-on

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) g++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.

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