Dependency for json-path added to test starter

Added `com.jayway.jsonpath` to `spring-boot-starter-test`.

JsonPath is mentioned in the documentation (http://docs.spring.io/spring/docs/current/spring-framework-reference/html/testing.html#spring-mvc-test-server) so I was surprised that using it causes:

```
Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.164 sec <<< FAILURE! - in demo.FooBarTest
testFooBar(demo.FooBarTest)  Time elapsed: 0.163 sec  <<< ERROR!
java.lang.NoClassDefFoundError: com/jayway/jsonpath/InvalidPathException
        at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
        at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
        at org.springframework.test.web.servlet.result.JsonPathResultMatchers.<init>(JsonPathResultMatchers.java:43)
        at org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath(MockMvcResultMatchers.java:183)
 ```

Added it to the starter so it's available out of the box


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