Mark android-support library as optional.

Projects that don't use the support library shouldn't have to install it
for robolectric to work. It should only be needed for projects that use
the support library shadows (and will have the support library available).

Closes #1087.
diff --git a/pom.xml b/pom.xml
index 780f5a2..4fa0e8b 100644
--- a/pom.xml
+++ b/pom.xml
@@ -148,6 +148,7 @@
       <artifactId>support-v4</artifactId>
       <version>19.0.1</version>
       <scope>compile</scope>
+      <optional>true</optional>
     </dependency>
 
     <dependency>