Add LaTeX to Travis CI install script

This commit adds a LaTeX installation to the Travis CI install script.
LaTeX is needed for the `SciencePlots` package, which uses it in
epispot's tests. This also (hopefully) fixes the error from the
previous commit on Travis CI.

Changelist:
- Add LaTeX requirements to `.travis.yml::install`

Additional Notes:
Fixes (major):
Fix `0ca85d13b579e624ca5dfff701a7a7c4c62a0d6c`

.travis.yml

@@ -12,9 +12,10 @@ before_install:
 install:
   - pip install numpy
   - pip install matplotlib
+  - pip install SciencePlots
+  - sudo apt-get install dvipng texlive-latex-extra texlive-fonts-recommended cm-super  # install LaTeX requirements for Travis CI
   - pip install pandas
   - pip install plotly
-  - pip install SciencePlots
   - pip install pytest
   - pip install coverage
   - pip install codecov