Reverts the fahrschein-http-simple to runtime dependency (#329)

Although technically correct (there is no dependency at build time),
we want to expose the SimpleRequestFactory to users, which is
prohibited by gradle's interpretation of the pom.xml

#328

fahrschein/pom.xml

@@ -19,7 +19,6 @@
             <groupId>org.zalando</groupId>
             <artifactId>fahrschein-http-simple</artifactId>
             <version>${project.version}</version>
-            <scope>runtime</scope>
         </dependency>
         <dependency>
             <groupId>org.zalando</groupId>