Add java8 runtime option to production appengine-web.xml files

It occurs to me that we can't have this setting different between sandbox
and production, otherwise we can end up with a situation where we push code
that works on sandbox but then fails only when it is pushed to production.
Sandbox and production need to always be set up similarly for this reason.

We'll just have to pay a greater amount of attention to the release process
next week than normal, and continue playing around in alpha for the mean
time with a fully Java 8 build.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170197703

java/google/registry/env/production/backend/WEB-INF/appengine-web.xml

@@ -3,6 +3,7 @@
 
   <application>domain-registry</application>
   <version>1</version>
+  <runtime>java8</runtime>
   <module>backend</module>
   <threadsafe>true</threadsafe>
   <sessions-enabled>true</sessions-enabled>

java/google/registry/env/production/default/WEB-INF/appengine-web.xml

@@ -3,6 +3,7 @@
 
   <application>domain-registry</application>
   <version>1</version>
+  <runtime>java8</runtime>
   <module>default</module>
   <threadsafe>true</threadsafe>
   <sessions-enabled>true</sessions-enabled>

java/google/registry/env/production/tools/WEB-INF/appengine-web.xml

@@ -3,6 +3,7 @@
 
   <application>domain-registry</application>
   <version>1</version>
+  <runtime>java8</runtime>
   <module>tools</module>
   <threadsafe>true</threadsafe>
   <sessions-enabled>true</sessions-enabled>