Fix java-pinning-core dependency in java-pinning-java7: use 'api'

Since Java7Pinning subclasses JavaPinning, consumers of
java-pinning-java7 need to java java-pinning-core in their compliation
classpath. Hence it is not sufficient ot use 'implementation', we need
to use 'api'.

Author: Flowdalic

java-pinning-java7/build.gradle

@@ -1,4 +1,4 @@
 dependencies {
-	implementation project(':java-pinning-core')
+	api project(':java-pinning-core')
 	testImplementation project(path: ':java-pinning-core', configuration: 'testRuntime')
 }