Require Racket 6.2

- Drop support for Rackets < 6.2 because they are no longer testable
  -- see <racket/racket-lang-org#88>.

- Add newer Racket version 7.2.

.travis.yml

@@ -19,17 +19,15 @@ env:
     # testing with newer Emacs.)                                     #
     ##################################################################
     # Emacs 24.3
-    - EMACS_VERSION=24.3 RACKET_VERSION=7.1
+    - EMACS_VERSION=24.3 RACKET_VERSION=7.2
     - EMACS_VERSION=24.3 RACKET_VERSION=HEAD
     # Emacs 24.5
-    - EMACS_VERSION=24.5 RACKET_VERSION=7.1
+    - EMACS_VERSION=24.5 RACKET_VERSION=7.2
     - EMACS_VERSION=24.5 RACKET_VERSION=HEAD
     # Emacs 25.1
-    - EMACS_VERSION=25.1 RACKET_VERSION=7.1
+    - EMACS_VERSION=25.1 RACKET_VERSION=7.2
     - EMACS_VERSION=25.1 RACKET_VERSION=HEAD
     # Emacs 26.1
-    - EMACS_VERSION=26.1 RACKET_VERSION=6.0
-    - EMACS_VERSION=26.1 RACKET_VERSION=6.1
     - EMACS_VERSION=26.1 RACKET_VERSION=6.2
     - EMACS_VERSION=26.1 RACKET_VERSION=6.3
     - EMACS_VERSION=26.1 RACKET_VERSION=6.5
@@ -42,12 +40,13 @@ env:
     - EMACS_VERSION=26.1 RACKET_VERSION=6.12
     - EMACS_VERSION=26.1 RACKET_VERSION=7.0
     - EMACS_VERSION=26.1 RACKET_VERSION=7.1
+    - EMACS_VERSION=26.1 RACKET_VERSION=7.2
     - EMACS_VERSION=26.1 RACKET_VERSION=HEAD
     - EMACS_VERSION=26.1 RACKET_VERSION=HEADCS
     # Emacs snapshot. Because slow to make, test against only some key
     # versions of Racket like latest stable and HEAD. Also list these
     # jobs below under `allow_failures`, and use `fast_finish`.
-    - EMACS_VERSION=snapshot RACKET_VERSION=7.1
+    - EMACS_VERSION=snapshot RACKET_VERSION=7.2
     - EMACS_VERSION=snapshot RACKET_VERSION=HEAD
 
 matrix:
@@ -59,7 +58,7 @@ matrix:
     - env: EMACS_VERSION=26.1 RACKET_VERSION=HEAD
     - env: EMACS_VERSION=26.1 RACKET_VERSION=HEADCS
     # bleeding edge Emacs
-    - env: EMACS_VERSION=snapshot RACKET_VERSION=7.1
+    - env: EMACS_VERSION=snapshot RACKET_VERSION=7.2
     - env: EMACS_VERSION=snapshot RACKET_VERSION=HEAD
   fast_finish: true