Add a .travis.yml file

Add the Travis-CI configuration file, which will allow our Gradle build to be
tested from Github with travis.

TESTED:
  1) Repeatedly tweaked this on own local fork of google/nomulus until I was able to successfully build.
  2) Verified that this shows up in the tarball after "blaze build :opensource-staging"

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219678030

.travis.yml

@@ -0,0 +1,45 @@
+# Copyright 2018 The Nomulus Authors. All Rights Reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+language: java
+install: true
+jdk:
+  # Our builds fail against Oracle Java for reasons yet unknown.
+  - openjdk8
+
+# Caching options suggested by a random article.
+before_cache:
+  - rm -f $HOME/.gradle/caches/modules-2/modules-2.lock
+  - rm -f $HOME/.gradle/caches/*/plugin-resolution/
+
+cache:
+  directories:
+    - $HOME/.gradle/caches/
+    - $HOME/.gradle/wrapper/
+
+env:
+  # Disable fancy status information (looks bad on travis and exceeds logfile
+  # quota)
+  TERM=dumb
+
+install:
+  # Install a specific gradle version first, default gradle can't deal with
+  # our gradle build scripts.
+  - wget http://services.gradle.org/distributions/gradle-4.10.2-bin.zip && unzip gradle-4.10.2-bin.zip
+
+# Specialize gradle build to use an up-to-date gradle and the /gradle
+# directory.
+# The "travis_wait 45" lets our build spend up to 45 minutes without writing
+# output, instead of the default 10.
+script: cd gradle && ../gradle-4.10.2/bin/gradle wrapper && travis_wait 45 ./gradlew build