Fix convert_markdown_into_ipynb script

* User could use convert tools in PATH

.travis.yml

@@ -19,7 +19,7 @@ before_install:
   -  pip install virtualenv pre-commit
   -  GOPATH=/tmp/go go get -u github.com/wangkuiyi/ipynb/markdown-to-ipynb
 script:
-  - travis/precommit.sh
+  -  PATH=/tmp/go/bin:$PATH travis/precommit.sh
 notifications:
   email:
     on_success: change

pre-commit-hooks/convert_markdown_into_ipynb.sh

 #!/bin/sh
 for file in $@ ; do
-	/tmp/go/bin/markdown-to-ipynb < $file > ${file%.*}".ipynb"
+    markdown-to-ipynb < $file > ${file%.*}".ipynb"
     if [ $? -ne 0 ]; then
         echo >&2 "markdown-to-ipynb $file error"
         exit 1