Use lein 2.6.1 on travis

to fix hack to use newer version of lein when only lein1 was available
on travis. no confusing exception anymore during environment setup.

follows up changes in clojure-emacs/cider-nrepl#419

Author: Benedek Fazekas

.travis.yml

@@ -1,15 +1,10 @@
 language: clojure
-install:
-  - wget -c https://raw.githubusercontent.com/technomancy/leiningen/2.5.3/bin/lein -O latest-lein
-  - chmod 755 latest-lein
-  - ./latest-lein
-before_script:
-  - ./latest-lein --version
+lein: 2.6.1
 script:
-  - ./latest-lein source-deps :prefix-exclusions "[\"classlojure\"]"
-  - ./latest-lein with-profile +plugin.mranderson/config test
-  - ./latest-lein with-profile +1.8,+plugin.mranderson/config test
-  - ./latest-lein with-profile +1.9,+plugin.mranderson/config test
+  - lein2 source-deps :prefix-exclusions "[\"classlojure\"]"
+  - lein2 with-profile +plugin.mranderson/config test
+  - lein2 with-profile +1.8,+plugin.mranderson/config test
+  - lein2 with-profile +1.9,+plugin.mranderson/config test
 jdk:
   - openjdk7
   - oraclejdk7