Ensure scheme extensions are treated as plain text

lib/linguist/mimes.yml

@@ -54,3 +54,4 @@ text/cache-manifest @manifest
 text/plain @cu,cxx
 text/x-nimrod @nim
 text/x-ocaml @ml,mli,mll,mly,sig,sml
+text/x-scheme @rkt,scm,sls,sps,ss

test/test_blob.rb

@@ -292,6 +292,9 @@ class TestBlob < Test::Unit::TestCase
 
     # https://github.com/googleapi/googleapi/blob/master/demos/gmail_demo/gmail.dpr
     assert_equal Language['Delphi'], blob("program.dpr").language
+
+    # https://github.com/dharmatech/agave/blob/master/demos/asteroids.sps
+    assert_equal Language['Scheme'], blob("asteroids.sps").language
   end
 
   def test_lexer

test/test_mime.rb

@@ -171,6 +171,7 @@ class TestMime < Test::Unit::TestCase
     assert Mime.text?('.sh')
     assert Mime.text?('.sig')
     assert Mime.text?('.sml')
+    assert Mime.text?('.sps')
     assert Mime.text?('.src')
     assert Mime.text?('.tcl')
     assert Mime.text?('.texi')