[Travis] Fix getting version from git tags

Travis clones the git repo with a shallow depth and the git describe
command cannot find the version tag. Setting depth to 1000 should be
enough but can be increased if required.

Refs:
  https://docs.travis-ci.com/user/customizing-the-build/#git-clone-depth
  https://stackoverflow.com/a/51727114/175584

.travis.yml

@@ -12,6 +12,10 @@ env:
   global:
     - DISPLAY=:99.0
 
+git:
+  # Set greater depth to get version from tags.
+  depth: 1000
+
 matrix:
   include:
     - name: Unit tests - Python 2
@@ -68,8 +72,6 @@ before_script:
     --make-pidfile --pidfile /tmp/custom_xvfb_99.pid \
     --exec /usr/bin/Xvfb -- :99 -ac -screen 0 1280x1024x16;
     fi"
-  # Create missing version file
-  - echo "2.0.0.dev0" > RELEASE-VERSION
 
 script:
   - tox -e $TOX_ENV