Fix Flyway Core vs. Maven plug-in mess

Fixes build.gradle having had the Flyway Maven plug-in instead of flyway-core.

Other than just “not being nice & proper & correct”, this mistake leads to a lot of Maven JARs unnecessarily ending up in the WAR and on the classpath (incl. e.g. in-IDE),  which in turn led to the errors below due to a slfj4j version mess (when using the upcoming  upcoming sweet new simple MifosWithDBServerApplication main()

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/home/vorburger/.gradle/caches/artifacts-26/filestore/ch.qos.logback/logback-classic/1.1.2/jar/b316e9737eea25e9ddd6d88eaeee76878045c6b2/logback-classic-1.1.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/home/vorburger/.gradle/caches/artifacts-26/filestore/org.slf4j/slf4j-nop/1.5.3/jar/36a3c886235cddd05e55a979cef549196740231a/slf4j-nop-1.5.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/home/vorburger/.gradle/caches/artifacts-26/filestore/org.slf4j/slf4j-jdk14/1.5.6/jar/cc383fbd07dd1826bbcba1b907bbdc0b5be627f1/slf4j-jdk14-1.5.6.jar!/org/slf4j/impl/StaticLoggerBinder.class]g
SLF4J: Found binding in [jar:file:/home/vorburger/.m2/repository/ch/qos/logback/logback-classic/1.1.2/logback-classic-1.1.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.

java.lang.AbstractMethodError: org.slf4j.impl.JDK14LoggerAdapter.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;[Ljava/lang/Object;Ljava/lang/Throwable;)V at org.apache.commons.logging.impl.SLF4JLocationAwareLog.info(SLF4JLocationAwareLog.java:159)