Tune sphinx_rtd_theme version

With some recent changes that are slightly backwards compatible, we were going
to bump the version of the RTD theme. We wanted to drop the current version
pattern for the semver scheme, but Sphinx's requirement on the RTD theme is
pinned at >=0.1,<0.2. For now, we're stuck releasing on the current scheme.

This relaxes the versioning requirement for the theme, with 2.0 being the
following release that could contain backwards incompatible API changes. We'd
likely wait to bump to 1.0 until the next Sphinx release, as to not break
experience for users.

Refs snide/sphinx_rtd_theme#244

setup.py

@@ -53,7 +53,7 @@ requires = [
     'snowballstemmer>=1.1',
     'babel>=1.3,!=2.0',
     'alabaster>=0.7,<0.8',
-    'sphinx_rtd_theme>=0.1,<0.2',
+    'sphinx_rtd_theme>=0.1,<2.0',
 ]
 extras_require = {
     # Environment Marker works for wheel 0.24 or later