Use the latest read the docs theme

Always pull the latest read the docs theme to receive
new features and bug fixes.

For example currently used version does not support [0]
code-block syntax highlight, which is used in the
Treasuremap documentation.

[1] https://github.com/rtfd/readthedocs.org/issues/4050

Change-Id: I58a096708b2741fc4af5d7701d276ea0551eca64

doc/requirements.txt

@@ -1,2 +1,2 @@
 sphinx>=1.6.2
-sphinx_rtd_theme==0.2.4
+sphinx_rtd_theme>=0.4.3