Copy some GWT settings from Guava to Truth.

The Truth GWT Travis build is failing with "GC overhead limit exceeded":
https://travis-ci.org/google/truth/builds/498829960

I don't know whether this CL will help or not. If not, the next thing to try is probably to define a module and set it with <module>, as we also do with Guava.

RELNOTES=n/a

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=235764378
diff --git a/core/pom.xml b/core/pom.xml
index d250fe6..a4e9b8c 100644
--- a/core/pom.xml
+++ b/core/pom.xml
@@ -156,7 +156,10 @@
             <goals><goal>test</goal></goals>
             <configuration>
               <mode>htmlunit</mode>
+              <htmlunit>FF38</htmlunit>
               <productionMode>true</productionMode>
+              <!-- Attempt to fix OutOfMemoryError in Travis. -->
+              <extraJvmArgs>-Xms3500m -Xmx3500m -Xss1024k</extraJvmArgs>
               <sourceLevel>auto</sourceLevel>
               <userAgents>gecko1_8</userAgents>
               <includes>**/*GwtTest.java</includes>