travis fix (#18)

* Dependency fix for travis.
* switching to python 3.7

Author: neelgala

.travis.yml

@@ -1,4 +1,6 @@
 language: python
+python:
+  - "3.7"
 sudo: required
 before_install:
   - sudo apt-get -qq update && sudo apt-get install -y --no-install-recommends texlive-fonts-recommended texlive-latex-extra texlive-fonts-extra dvipng texlive-latex-recommended latexmk

docs/requirements.txt

@@ -1,33 +1,33 @@
-alabaster==0.7.12
-Babel==2.7.0
-Cerberus==1.3.1
-certifi==2019.6.16
-chardet==3.0.4
-doc8==0.8.0
-docutils==0.14
-gitdb2==2.0.5
-idna==2.8
-imagesize==1.1.0
-Jinja2==2.10.1
-MarkupSafe==1.1.1
-oyaml==0.9
-packaging==19.0
-pbr==5.3.1
-Pygments==2.4.2
-pyparsing==2.4.0
-python-dateutil==2.8.0
-pytz==2019.1
-PyYAML==5.1.1
-requests==2.22.0
-restructuredtext-lint==1.3.0
-ruamel.yaml==0.15.97
-six==1.12.0
-smmap2==2.0.5
-snowballstemmer==1.2.1
-Sphinx==1.7.5
-sphinx-rtd-theme==0.4.3
-sphinxcontrib-autoyaml==0.5.0
-sphinxcontrib-websupport==1.1.2
-stevedore==1.30.1
-urllib3==1.25.3
-twine==1.13.0
+alabaster>=0.7.12
+Babel>=2.7.0
+Cerberus>=1.3.1
+certifi>=2019.6.16
+chardet>=3.0.4
+doc8>=0.8.0
+docutils>=0.14
+gitdb2>=2.0.5
+idna>=2.8
+imagesize>=1.1.0
+Jinja2>=2.10.1
+MarkupSafe>=1.1.1
+oyaml>=0.9
+packaging>=19.0
+pbr>=5.3.1
+Pygments>=2.4.2
+pyparsing>=2.4.0
+python-dateutil>=2.8.0
+pytz>=2019.1
+PyYAML>=5.1.1
+requests>=2.22.0
+restructuredtext-lint>=1.3.0
+ruamel.yaml>=0.15.97
+six>=1.12.0
+smmap2>=2.0.5
+snowballstemmer>=1.2.1
+Sphinx>=1.7.5
+sphinx-rtd-theme>=0.4.3
+sphinxcontrib-autoyaml>=0.5.0
+sphinxcontrib-websupport>=1.1.2
+stevedore>=1.30.1
+urllib3>=1.25.3
+twine>=1.13.0