Update drone.yml

drone.yml

     - apt-get --yes upgrade
     - apt-get --yes install $BUILDDEPS
     - export MAKEFLAGS
-    - cd src && make 
-    - cd contrib && make 
-    - cd .. ; ls -l radangel contrib/websockify ; md5sum radangel contrib/websockify ; cd ..
+    - make -C src
+    - make -C src/contrib
+    - ls -l  src/radangel src/contrib/websockify
+    - md5sum src/radangel src/contrib/websockify
   when:
     branch:
     - master