Add manual trigger to TravisCI.
This change adds the ability for Integrations to run TravisCI manually.

A checkbox is added to the TravisCI form that indicates if TravisCI should

only run manually.
This change also fixes the Jenkins build failing for git repositories with less

than 50 commits since git fetch --unshallow origin fails.
There was a bug where no TravisCI builds can be made on personal private GitHub

repositories because the GitHub username was being extracted from the wrong place.
Based on work by David Trowbridge: /r/11130

Manually tested on Review Board 3. TravisCI (travis-ci.com) was set up and 

connected to a private GitHub repository and Review Board 3. A TravisCI build

was not run when the manually run checkbox is unchecked and is run when the

field is checked. Clicking on the 'Run' button runs the TravisCI build.
Verified the disabled/enabled run manually field by testing on Review Board

3.0.18, 3.0.19 and 4.0.x.
Verified retry functionality when state is in ERROR and TIMEOUT states.
Unit tests to verify that a build is not run when the run manually

configuration is checked and to verify that a build is run when the signal is

emitted.
Unit tests were also created to verify that the username is being pulled

from the correct place for the 4 different GitHub repository plans.

These were ran on Review Board 3.0 and Review Board 4.0 and their

supported versions of Python.