Setup cron job to test against dependencies from source

[rahulporuri]

Travis (and possibly appveyor too) allow the creation of cron jobs. Some of the ETS packages now have cron jobs setup to test against dependencies which are installed from source - instead of testing against the released versions.

See examples :

• Test ETS packages from source using cron jobs  envisage#162
• Add cron job to install/test against packages from source traitsui#914
• Add cron job to install/test against packages from source pyface#549