Release: don't build Python 3 tarball

Just build one tar.gz, it should work with all Pythons.
Add numeric computation doc page

Discusses lambdify, ufuncify, and Theano options for numeric evaluation
add link to numeric computation in modules index
replace x.length with n
add evalf
The minimal polynomial of products of rational powers of integers are
computed much faster in some cases.

In master

``minimal_polynomial(nsimplify(pi, tolerance=0.000000001))``

hangs (see #2234 and issue 4014)

Now it takes a fraction of a second.
added missing import
Blacklist numeric-computation.rst tests if numpy or theano modules are not imported
Fix tests in numeric-computation.rst
Install numpy for numeric-computation.rst if TEST_THEANO is true
Merge pull request #13 from skirpichev/mrocklin-computation-docs

fix travis failures
Fix sphinx test in travis: drop evalf-label
Install scipy in travis (TEST_THEANO) for numeric-computation.rst
Fix trailing whitespaces and reformat long lines
Merge pull request #14 from skirpichev/mrocklin-computation-docs

More fixes
Install scipy deps, including blas, gfortran, etc
Merge pull request #15 from skirpichev/mrocklin-computation-docs

Try to install scipy deps, including blas, gfortran, etc
semicolons!
Merge pull request #16 from skirpichev/mrocklin-computation-docs

fix travis failures
Use print functions in fabfile.py