Upgrade reqwest and pulldown-cmark dependencies

[mati865]

About the changes:

• use_sys_proxy is now default
• somehow serde stops working with new reqwest unless derive feature is enabled
• codespan is left with old version because I didn't know how to solve breaking changes

[Michael-F-Bryan]

@mati865 it looks like the travis build failed, any idea what might have caused it?

[mati865]

I think Travis has changed their images to not include openssl by default any more.
To confirm that I triggered build for current master branch and it failed the same way: https://travis-ci.com/mati865/mdbook-linkcheck/builds/150833470

You can install openssl-dev package on the CI to solve it.

[palfrey]

#29 fixes the build issues

[Michael-F-Bryan]

@mati865, would you be able to do a git rebase and see if CI is happy again?

[mati865]

It's green now.