fix shell keywords and bugs

pre-commit-hooks/convert_markdown_into_ipynb.sh

 #!/bin/sh
 command -v go >/dev/null 2>&1
-if [ $? != 0 ]; then
-    echo -e >&2 "Please install https://golang.org/doc/install#install"
+if [[ $? != 0 ]]; then
+    echo >&2 "Please install https://golang.org/doc/install#install"
     exit 1
 fi
 
@@ -10,8 +10,8 @@ GOPATH=/tmp/go go get -u github.com/wangkuiyi/ipynb/markdown-to-ipynb
 
 for file in $@ ; do
 	/tmp/go/bin/markdown-to-ipynb < $file > ${file%.*}".ipynb"
-    if [ $? != 0 ]; then
-        echo -e "markdown-to-ipynb $file error"
+    if [[ $? != 0 ]]; then
+        echo >&2 "markdown-to-ipynb $file error"
         exit 1
     fi
 done

tools/convert-markdown-into-ipynb-and-test.sh

 #!/bin/sh
 command -v go >/dev/null 2>&1
-if [ $? != 0 ]; then
+if [[ $? != 0 ]]; then
     echo >&2 "Please install go https://golang.org/doc/install#install"
     exit 1
 fi
@@ -13,13 +13,13 @@ cd $cur_path/../
 #convert md to ipynb
 for file in */{README,README\.en}.md ; do
     /tmp/go/bin/markdown-to-ipynb < $file > ${file%.*}".ipynb"
-    if [ $? != 0 ]; then
+    if [[ $? != 0 ]]; then
         echo >&2 "markdown-to-ipynb $file error"
         exit 1
     fi
 done
 
-if [[ ! -z $TEST_EMBEDDED_PYTHON_SCRIPTS ]]; then
+if [[ -z $TEST_EMBEDDED_PYTHON_SCRIPTS ]]; then
     exit 0
 fi