[util] fix bug with backtraces (only happened with Ruby < 2.2)

Author: prettycoder

lib/appoptics_apm/api/util.rb

@@ -45,7 +45,7 @@ def trim_backtrace(backtrace)
         return backtrace unless backtrace.is_a?(Array) && backtrace.size > BACKTRACE_CUTOFF
 
         # Trim backtraces by getting the first 180 and last 20 lines
-        backtrace[0, 180] + ['...[snip]...'] + backtrace[length - 20, 20]
+        backtrace[0, 180] + ['...[snip]...'] + backtrace[backtrace.size - 20, 20]
       end
 
       # Internal: Check if a host is blacklisted from tracing