Fix attemptd assignment to non-variable in bash 3 (spf13#628)

* Fix attemptd assignment to non-variable in bash 3

flaghash variable is an associative array which is only supported in
bash > 3.

* Use -gt instead of >

bash_completions.go

@@ -175,12 +175,15 @@ __handle_flag()
     fi
 
     # keep flag value with flagname as flaghash
-    if [ -n "${flagvalue}" ] ; then
-        flaghash[${flagname}]=${flagvalue}
-    elif [ -n "${words[ $((c+1)) ]}" ] ; then
-        flaghash[${flagname}]=${words[ $((c+1)) ]}
-    else
-        flaghash[${flagname}]="true" # pad "true" for bool flag
+    # flaghash variable is an associative array which is only supported in bash > 3.
+    if [[ -z "${BASH_VERSION}" || "${BASH_VERSINFO[0]}" -gt 3 ]]; then
+        if [ -n "${flagvalue}" ] ; then
+            flaghash[${flagname}]=${flagvalue}
+        elif [ -n "${words[ $((c+1)) ]}" ] ; then
+            flaghash[${flagname}]=${words[ $((c+1)) ]}
+        else
+            flaghash[${flagname}]="true" # pad "true" for bool flag
+        fi
     fi
 
     # skip the argument to a two word flag