Turn warnings into errors
Fix dialyzer warnings due to -Wunmatched_returns option.

Below are the warnings that were fixed by this commit.

[git: master]Iris:ibrowse andreas$ make dialyzer
Building local plt at .dialyzer_plt

dialyzer --output_plt .dialyzer_plt --build_plt \
	   --apps erts kernel stdlib ssl crypto public_key asn1 compiler hipe edoc gs syntax_tools inets xmerl runtime_tools mnesia
  Compiling some key modules to native code... done in 0m19.73s
  Creating PLT .dialyzer_plt ... done in 1m40.98s
done (passed successfully)
dialyzer --fullpath --plt .dialyzer_plt -Wrace_conditions -Wunmatched_returns -Werror_handling -r ./ebin
  Checking whether the PLT .dialyzer_plt is up-to-date... yes
  Proceeding with analysis...
src/ibrowse_http_client.erl:197: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:298: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:322: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:335: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:352: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:582: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:587: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:590: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:706: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:789: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:797: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:1402: Expression produces a value of type 'ok' | {'error',_}, but this value is unmatched
src/ibrowse_http_client.erl:1859: Expression produces a value of type 'ok' | integer(), but this value is unmatched
src/ibrowse_socks5.erl:25: Expression produces a value of type {'ok',<<_:16>>}, but this value is unmatched
 done in 0m8.07s
done (warnings were emitted)
make: *** [dialyzer] Error 2
Run dialyzer by default just after a compile
Add R16B to travis versions
Remove OTP versions from that seem not to be supported by travis anymore
Generating a dialyzer PLT on travis with R14 takes more than 10 minutes, and then travis kills the build
Add R15B to travis
Remove R15B from travis, it also takes more than 10 minutes to generate a dialyzer PLT file
Merge pull request #103 from apauley/dialyzer_travis

Run dialyzer by default when compiling. Update travis builds.