Update .travis.yml to run tests on Racket version 6.3 and 6.12, the e…

…arliest and latest versions that are expected to work right now. Describe why previous versions don't work.

.travis.yml

@@ -11,9 +11,29 @@ env:
     - RACKET_DIR=~/racket
     - TRAVIS_RACKET_DIR=~/travis-racket
   matrix:
-#    - RACKET_VERSION=6.6
-#    - RACKET_VERSION=6.7
-    - RACKET_VERSION=6.8
+
+    # Versions prior to 5.92 don't seem to install the current
+    # directory as a package named "anarki" using
+    # `raco pkg install --deps search-auto`. It's unknown how well
+    # they work.
+
+    # Versions prior to 6.0.1 don't provide the `--drdr` option, which
+    # is used by one of the testing commands here, but they otherwise
+    # should work as well as the versions after.
+
+    # Versions prior to 6.1 don't provide the `--check-pkg-deps`
+    # option, which is used by one of the testing commands here, but
+    # they otherwise should work as well as the versions after.
+
+    # Versions prior to 6.3 don't define the `racket/random` module,
+    # which is needed for news.arc and one of the tests.
+
+    # This is the earliest known supported version.
+    - RACKET_VERSION=6.3
+
+    # This is the latest known supported version.
+    - RACKET_VERSION=6.12
+
 #    - RACKET_VERSION=HEAD
 
 matrix: