Build binary when package version < 0.2

[mamedin]

Acmetool v0.2 official packages are not available. This commits allows to
build the v0.2 binary from sources when acmetool package version < 0.2.

Connects to #17

[djjuhasz]

I still think we should be using certbot and https://github.com/geerlingguy/ansible-role-certbot to deploy Let's Encrypt certificates.

[hakamine]

I have the following concerns:

• The added build tasks are not idempotent (the role will attempt to build the binary each time it is run on a server as it checks acmetool version from the installed package)
• We will need to revert the changes made by this PR when the acmetool maintainers release the packages for v0.2
• It adds a golang build environment to the target server

I was wondering if we could just add a note to the README, with instructions on how to build the binary (e.g., here), mentioning this is a required temporary workaround until the acmetool maintainers release the packages for the new version (and also that the package needs to be built only once and then copied to any server as required, no need to build several times on each server)

[luisilara]

I like the idea of using certbot, but this fix also LGTM!