Disable --pre flag in tox installs to fix environment recreation.

Remove an old hack for monotime package.
Add coveralls to travis config.
Disable coverage collection on python 2.6.
Correctly skip coverage collection on 2.6.
Remove 'cd maint' from travis script.

This was needed for 2to3, which we no longer use.  The changed
directory meant that coveralls wasn't seeing our .coveragerc.
Fix doc build on travis after removal of 'cd maint'