Prefer let x+=1 for incrementing counters

The `((x++))` syntax is shorthand for `let x++`. According to `help let`:

    If the last ARG evaluates to 0, let returns 1; 0 is returned
    otherwise.

Thus the exit status of the expression `x=0; let x++` is 1, since the post-increment `++` operator evaluates to the value of the variable before incrementing.

In Bash 4, this non-zero exit status properly triggers `set -e`'s error trap, but in Bash 3 it does not. That's why the tests were passing on OS X (Bash 3) but not Linux (Bash 4).

We can work around the problem by choosing an incrementation expression that never evaluates to 0, such as `+=` or the pre-increment `++` operator. For consistency and clarity, I've changed to `x+=1` everywhere.

Ref. #25, #27

libexec/bats-format-tap-stream

@@ -131,7 +131,7 @@ trap finish EXIT
 while IFS= read -r line; do
   case "$line" in
   "begin "* )
-    index=$(( $index + 1 ))
+    let index+=1
     name="${line#* $index }"
     buffer begin
     flush
@@ -145,11 +145,10 @@ while IFS= read -r line; do
     fi
     ;;
   "not ok "* )
-    failures=$(( $failures + 1 ))
+    let failures+=1
     buffer fail
     ;;
   "# "* )
-
     buffer log "${line:2}"
     ;;
   esac