ptyprocess API changed to a single preexec_fn function
Allow the user to specify a preexec_fn, which will be passed to ptyprocess
Update test case for preexec_fn
Merge pull request #158 from takluyver/preexec_fn

Expose preexec_fn from ptyprocess
Speed up Travis builds

Skip installing apt python-yaml package, pip reinstalls it anyway.

Use new Travis containerised stack.
Merge pull request #165 from takluyver/faster-travis

Speed up Travis builds
Merge remote-tracking branch 'origin/master' into issue-104-cannot-exec-setuids