Clone tag only

There were merge conflict issues appearing with cloning first and then
checking out (although it shouldn't). Cloning tag only might solve it.
And anyways that should really be the preferred way, it's faster and
downloads only the part we're interested in.

.travis.yml

@@ -80,15 +80,11 @@ before_install:
     - pip install --upgrade -r requirements.txt
     # Checkout libres - need libecl version
     - source .libres_version
-    - git clone https://github.com/equinor/libres
+    - git clone --branch $LIBRES_VERSION --depth 1 https://github.com/equinor/libres
     - pushd libres
-    - git checkout tags/$LIBRES_VERSION
     - source .libecl_version
     - popd
-    - git clone https://github.com/equinor/libecl
-    - pushd libecl
-    - git checkout tags/$LIBECL_VERSION
-    - popd
+    - git clone --branch $LIBECL_VERSION --depth 1 https://github.com/equinor/libecl
 
 install:
   # For now we have to make install libecl.