rebar.config: Update PropEr from 1.4.0 to 1.5.0

[Why]
This should fix a deprecation warning reported at compile time for quite
some time.

Author: dumbbell

rebar.config

@@ -52,7 +52,7 @@
 
 {profiles,
  [{test,
-   [{deps, [{proper, "1.4.0"},
+   [{deps, [{proper, "1.5.0"},
             %% FIXME: We need to add `cth_readable' as a dependency and an
             %% extra app for Dialyzer. That's because Rebar is using that
             %% application to override `ct:pal()' and Dialyzer complains it