Rework .travis.yml to use wheels.
Disable TravisCI email notifications.
Add handy hyperlink from tutorial to DiGraph class
Merge pull request #1139 from matt-hickford/patch-1

Add handy hyperlink from tutorial to DiGraph class
Add pygraphviz to .travis.yml
Update test_threshold.py

Fixes #1140
Install pygraphviz only for 2.x
Merge pull request #1129 from chebee7i/wheels

Use wheels with TravisCI for NumPy, etc. Closes #1137.
Seed random number geneator for edge swap tests.
Fix check for PyGraphviz in .travis.yml
Fix check for PyGraphviz in .travis.yml (cont'd)
Fix check for PyGraphviz in .travis.yml (cont'd)
Simplify .travis.yml
Merge pull request #1138 from chebee7i/seedswap

Fix nondeterministic unit tests for double-edge swap.