Correct csh syntax in activate_virtualenv

In csh, the . operator executes a directory, which is the wrong
behaviour here. At best, when pipenv is installed system-wide
(preferably via the operating system package manager), the result
is a permission denied error from the shell.

As csh uses the source command, reflect that here.

pipenv/core.py

@@ -1218,6 +1218,7 @@ def activate_virtualenv(source=True):
     # Support for csh shell.
     if PIPENV_SHELL and 'csh' in PIPENV_SHELL:
         suffix = '.csh'
+        command = 'source'
 
     # Escape any spaces located within the virtualenv path to allow
     # for proper activation.