Remove transitive dependency on logback-classic

[msgilligan]

When I include this library in my project (indirectly through 'org.multibit.hd.hardware:trezor:0.7.0'), slf4j gets confused when finding logback-classic on the classpath. It reports:

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/path/slf4j-jdk14-1.7.21.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/path/logback-classic-1.1.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.JDK14LoggerFactory]

A library JAR that uses slf4j shouldn't require (transitively include) a particular binding. My workaround is to use the Gradle exclude module: option:

compile ('org.multibit.hd.hardware:trezor:0.7.0') {
    exclude module: "logback-classic"
}