make license check slightly more lenient

bin/spack now needs to have a "-*- python -*-" line after the shebang, so
that emacs will interpret it as a python file instead of as a shell
script.  Add one line to the license check limit to accommodate this.

lib/spack/spack/cmd/license.py

@@ -22,7 +22,7 @@
 git = which('git')
 
 #: SPDX license id must appear in the first <license_lines> lines of a file
-license_lines = 6
+license_lines = 7
 
 #: Spack's license identifier
 apache2_mit_spdx = "(Apache-2.0 OR MIT)"