travis: add dependencies for plantuml

Adds the missing dependencies for plantuml.

This includes a workaround for the issue reported in
https://github.com/sphinx-contrib/plantuml/issues/56. Once the fix has
been included in a release we can simplify according to the comment in
.travis.yml.

Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/.travis.yml b/.travis.yml
index 38e7807..cf6bed2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -8,9 +8,13 @@
 
 before_install:
   - sudo apt-get update
-  - sudo apt-get install -y graphviz
+  - sudo apt-get install -y graphviz plantuml
 
-install: pip install sphinx sphinx_rtd_theme
+# TODO make this "pip install sphinx sphinx_rtd_theme sphinxcontrib-plantuml"
+# once the fix in https://github.com/sphinx-contrib/plantuml/issues/56 has
+# been included in a release.
+install:
+  - pip install sphinx==1.8.5 sphinx_rtd_theme sphinxcontrib-plantuml Pillow
 
 notifications:
   - email: false