Fix running of specs with rspec executable

Legacy specs should be filtered out by default when using `rspec`
executable with no arguments or a path that contains both legacy and
non-legacy specs (e.g. `rspec spec`).

As noted in 1a0e2b90, suites must be run separately for time-being.

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>

.rspec

@@ -27,3 +27,4 @@
 #++
 
 --color
+--exclude-pattern spec/legacy/**/*_spec.rb