Allows the user (or CI) to grab gdal from osgeo instead, if available (#162)

* Allows the user (or CI) to grab gdal from osgeo instead, if available

* Fixing the imported package

* The import error was causing travis-CI to fail.

* Reverting old PR edits and moving attempted oseo import to init

plio/io/__init__.py

@@ -13,7 +13,11 @@ try:
     osr = osr.loader.load_module()
     gdal.UseExceptions() 
 except:
-    gdal = None
+    try:
+        gdal = importlib.util.find_spec('osgeo.gdal')
+        gdal = gdal.loader.load_module()
+    except:
+        gdal = None
     ogr = None
     osr = None