Update HTML documentation to point to Guava 15 instead of 14.

Test Plan: Not tested.
diff --git a/docs/contributing/codestyle.soy b/docs/contributing/codestyle.soy
index 4e13c19..6386073 100644
--- a/docs/contributing/codestyle.soy
+++ b/docs/contributing/codestyle.soy
@@ -29,13 +29,13 @@
       <li>In the spirit of Guava, references are assumed to be
           non-null unless otherwise noted.
       <li>Non-null parameters should be verified using
-          {sp}<a href="{GUAVA_URL}
+          {sp}<a href="{GUAVA_BASE_URL}
           com/google/common/base/Preconditions.html#checkNotNull(T)">
           <code>Preconditions.checkNotNull()</code></a>.
       <li>A nullable reference should be annotated with
           {sp}<code>@javax.annotation.Nullable</code>.
       <li>A popular alternative to <code>@Nullable</code> is
-          {sp}<a href="{GUAVA_URL}
+          {sp}<a href="{GUAVA_BASE_URL}
           com/google/common/base/Optional.html">
           <code>com.google.common.base.Optional</code></a>.
       <li>Annotating an <code>Optional</code> as <code>@Nullable</code> is
diff --git a/docs/globals.json b/docs/globals.json
index b54fa39..32013f1 100644
--- a/docs/globals.json
+++ b/docs/globals.json
@@ -4,7 +4,7 @@
   "GITHUB_URL" : "https://www.github.com/facebook/buck/",
 
   // The value must always end with a forward slash.
-  "GUAVA_URL" : "http://docs.guava-libraries.googlecode.com/git-history/v14.0/javadoc/",
+  "GUAVA_BASE_URL" : "http://docs.guava-libraries.googlecode.com/git-history/v15.0/javadoc/",
 
   // The value must always end with a forward slash.
   "GEN_DIR" : "buck-out/gen/",