Set PIP_DISABLE_PIP_VERSION_CHECK=1 after showing pip version
Set PIP_DISABLE_PIP_VERSION_CHECK=1 after showing pip version

Resolves https://github.com/travis-ci/travis-ci/issues/5499

By setting this value, we avoid making extra network calls
for each invocation of `pip`.
We do this *after* announcing the pip version, so that the user
may become aware.

craigcitro/travis-build,JuliaCI/travis-build,jhass/travis-build,akoeplinger/travis-build,jhass/travis-build,kevmoo/travis-build,kevmoo/travis-build,andyli/travis-build,andyli/travis-build,craigcitro/travis-build,akoeplinger/travis-build,JuliaCI/travis-build