oce: fix fetching (#2921)

* oce: fix fetching

apparently the fetcher is confused when there is a mixture of 3-digits
versions and 2-digits versions, i.e. 0.18 and 0.17.2

* flake8

var/spack/repos/builtin/packages/oce/package.py

@@ -32,7 +32,6 @@ class Oce(Package):
     Open CASCADE library.
     """
     homepage = "https://github.com/tpaviot/oce"
-    url      = "https://github.com/tpaviot/oce/archive/OCE-0.18.tar.gz"
 
     version('0.18',   '226e45e77c16a4a6e127c71fefcd171410703960ae75c7ecc7eb68895446a993')
     version('0.17.2', 'bf2226be4cd192606af677cf178088e5')
@@ -47,6 +46,10 @@ class Oce(Package):
     depends_on('cmake@2.8:', type='build')
     depends_on('tbb', when='+tbb')
 
+    def url_for_version(self, version):
+        return 'https://github.com/tpaviot/oce/archive/OCE-%s.tar.gz' % (
+            version.dotted)
+
     # There is a bug in OCE which appears with Clang (version?) or GCC 6.0
     # and has to do with compiler optimization, see
     # https://github.com/tpaviot/oce/issues/576