Update to latest build range number for 2019.1

Since the plugin is compatible with the latest UI theme API added in
Product version 2019.1, the `since` build range number (1) must match.

Therefore the number has been increased to `191.0` to fullfil the
minimum required version.

References:
  (1) http://www.jetbrains.org/intellij/sdk/docs/basics/getting_started/build_number_ranges.html

Resolves GH-54

resources/META-INF/plugin.xml

@@ -11,7 +11,7 @@
   </description>
 
   <!-- See http://www.jetbrains.org/intellij/sdk/docs/basics/getting_started/build_number_ranges.html for details.-->
-  <idea-version since-build="173.0"/>
+  <idea-version since-build="191.0"/>
 
   <!--
   See http://www.jetbrains.org/intellij/sdk/docs/basics/getting_started/plugin_compatibility.html on how to target