Fix #242: Update default Clojure version to 1.7.0

- This will not automatically update existing boot installations. This
  only changes the default Clojure version when first installing boot.

- To update an existing installation is still as described in the wiki[1],
  for example:

      $ BOOT_CLOJURE_VERSION=1.7.0 boot -u

[1]: https://github.com/boot-clj/boot/wiki/Keeping-Boot-Up-to-Date

boot/base/src/main/java/boot/App.java

@@ -60,7 +60,7 @@ public static class Exit extends Exception {
         String             c = cljversion;
         String             t = null;
 
-        if (c == null) c = "1.6.0";
+        if (c == null) c = "1.7.0";
 
         if (bootversion != null)
             p.setProperty("BOOT_VERSION", bootversion);

boot/core/src/boot/task/built_in.clj

@@ -29,7 +29,7 @@
     (let [tasks (#'helpers/available-tasks 'boot.user)
           opts  (->> main/cli-opts (mapv (fn [[x y z]] ["" (str x " " y) z])))
           envs  [["" "BOOT_AS_ROOT"            "Set to 'yes' to allow boot to run as root."]
-                 ["" "BOOT_CLOJURE_VERSION"    "The version of Clojure boot will provide (1.6.0)."]
+                 ["" "BOOT_CLOJURE_VERSION"    "The version of Clojure boot will provide (1.7.0)."]
                  ["" "BOOT_HOME"               "Directory where boot stores global state (~/.boot)."]
                  ["" "BOOT_FILE"               "Build script name (build.boot)."]
                  ["" "BOOT_JAVA_COMMAND"       "Specify the Java executable (java)."]