Update INSTALL with better instructions.

Remove explicit references to version 3.1 except for name of `propka31`
script which will be changed in subsequent commit.

INSTALL

@@ -3,9 +3,10 @@
 Clone repository or unpack the tar ball and install with
 [pip](https://pip.pypa.io):
 
-    pip install propka-3.1/
+    pip install <source directory>
 
 This will install the `propka31` script in your executable
-directory. The `propka` Python module will also become available for
-import.
+directory.  The `propka` Python module will also become available for import
+and can be invoked in lieu of the `propka31` script via:
 
+    python -m propka