Update HikariCP to 2.1.0, compile against the Java 6-compatible artifact

For those running applications on Java 8, dependency management for the
Java 8-compatible artifact is also provided.

Closes gh-1721
Fix ArrayStoreException caused by JerseyAutoConfiguration again

This is a follow-on from the changes made in 2b7bf3e. In addition to the
problematic use of @ConditionalOnClass that was addressed in 2b7bf3e,
JerseyAutoConfiguration also used @ConditionalOnBean referencing a 
Jersey class. This has the same problem when used on a class that
implements WebApplicationInitializer. Implementing
WebApplicationInitializer causes the class’s annotations to be
introspected during servlet container initialiser processing. If a
@ConditionalOnBean annotation references a Class that cannot be
loaded an ArrayStoreException occurs.

This commit updates JerseyAutoConfiguration to reference ResourceConfig
as a String. This allows it annotations to be introspected without
attempting to load a Jersey class that may not be on the classpath.

Fixes gh-1733
Fixes gh-1719
Protect against race condition where output file exists but it is empty

Previously, there was a timing window where the output file had been
created but it was empty. This would cause the test to fail as the
output was read from the empty file and didn’t match the expected “Hello
World”.

This commit updates the test to only process the resources in the output
directory when all the resolved resources have a non-zero content
length. An @Before method has also been added to delete the output
produced by the test so that the outcome of the test isn’t affected by
files generated by previous runs.

Fixes gh-1735
Merge branch '1.1.x'
Install SLF4JBridgeHandler when Log4J2LoggingSystem is initialized

Previously, Log4J2LoggingSystem did not install SLF4JBridgeHandler.
SLF4JBridgeHandler is used to route logging calls made using
java.util.logging into SLF4J. This resulted in entries being written
to System.err instead.

This commit updates Log4J2LoggingSystem to register the bridge handler
thereby ensuring the logging that originates into java.util.logging is
routed into SLF4J. This change has been made by extracting a new
abstract superclass that performs the registration. This super class is
extended by all SLF4J-based LoggingSystem implementations.

Fixes gh-1737
Update WebSocket sample with new SockJS CDN

Closes gh-1740
Merge branch '1.1.x'
Fix ConnectionFactory auto-configuration with XA

Previously, the regular jms connection factory was always overriding
the xaConnectionFactory when using ActiveMQ. While
ActiveMQAutoConfiguration is namely shielded with a condition on a
missing ConnectionFactory bean, said configuration class also imports the
XA and regular configuration classes that both can create the connection
factory.

This commit adds a ConditionalOnMissingBean in the second class that is
imported in case the XA configuration has already defined what it needs.

Fixes gh-1727
Polish
Merge branch '1.1.x'
Polish
Rework @TestPropertySource compatibility

Rework @TestPropertySource changes introduced in commit d251b513
to restore compatibility with Spring Boot 1.1

- Only add the `server.port` property when no @IntegrationTest
  annotation is found.
- Always add a default `spring.jmx.enabled=false` property.
- Restore the SpringApplicationContextLoader.getEnvironmentProperties
  protected method.
- Remove the @IntegrationTest.properties attribute.

See gh-1697
Upgrade to Reactor 1.1.5.RELEASE

Closes gh-1742
Merge branch '1.1.x'
Upgrade to 2.1.11 of the Git commit Maven plugin

Closes gh-1708
Merge branch '1.1.x'
Add Jade4J Starter to the starters README

Closes gh-1748
Closes gh-1703