[#89] Move problems under "exercises" dir

With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

bin/build.sh

@@ -1,10 +1,11 @@
 #!/bin/bash
 set -e
+pushd exercises
 gradle --version
 echo ""
 echo ">>> Running tests..."
 TERM=dumb gradle check compileStarterSourceJava --continue
-
+popd
 
 echo ""
 echo ">>> Running configlet..."