Fix expansion of $file

The message warning of a missing shebang! line clearly intended to
output which file was missing the shebang, but instead it accidentally
invoked the file(1) command with no arguments, resulting in an error.
Merge pull request #30 from aspiers/file-bug

Fix expansion of $file
use $< and $@ to avoid duplication

Follow the DRY rule: if the rule's target or dependency change, it's
less error-prone to avoid requiring that it be changed everywhere it's
used.
remove superfluous check from install-markdown

shocco is not required in order for the install-markdown target to be
built.
make shocco a proper dependency of the `install' target
always echo message when shocco has been built

If we run `make install', it should still show that shocco
was rebuilt.