Updated repositories in build.gradle for Maven Central

The build will complete with these gradle repository changes.

Unfortunately I still get known problem that SpringShell launches instead of RestShell. I tried to build the jbrisbin repo spring-shell instead of the offcial one, but I got an error during the gradlew build.

```
junit.framework.AssertionFailedError: Number of Converters is incorrect expected:<16> but was:<11>
	at junit.framework.Assert.fail(Assert.java:50)
	at junit.framework.Assert.failNotEquals(Assert.java:287)
	at junit.framework.Assert.assertEquals(Assert.java:67)
	at junit.framework.Assert.assertEquals(Assert.java:199)
	at org.springframework.shell.BootstrapTest.test(BootstrapTest.java:21)
```
Even after forcing the test to pass, the SpringShell launches instead of RestShell problem still exists.