.travis.yml: pip install from wheels to avoid lengthy compilations
MAINT: remove unnecessary things from .travis.yml
Merge pull request #3020 from  msabramo/travis_use_wheels

MAINT: .travis.yml: pip install from wheels to avoid lengthy compilations
MAINT: runtests: add option for gcov
Merge pull request #3021 from pv/gcov

MAINT: runtests: add option for gcov