Add missing dep examples and fix existing ones
deps: remove undocumented fossil syntax

As discussed with Martin Schut, remove support for
{app_name, "vsn", {fossil, "url", latest}.
Fix false reporting of (plain) vsn strings

rebar used to mistakenly report plain version strings like
{vsn, "1.0.0"} as follows:
DEBUG: vcs_vsn: Unknown VCS atom in vsn field: "1.0.0"

Properly detect unknown/unsupported version terms and abort
if we encounter one.

While at it, rename a variable in vcs_vsn/3 to be non-misleading.
erlc: properly reuse the right erl_opts in test_compile

Avoid getting erl_opts twice in test_compile/3, and make sure the
correctly filtered (platform_define, etc.) version is used.
lfe: use correctly filtered erl_opts
rebar_core: fix Dialyzer warning introduced in aa46d85 (#157)
Merge pull request #152 from tuncer/test-erl_opts

Fix erl_opts use
Merge pull request #172 from tuncer/dep-examples

Add missing dep examples and fix existing ones
Merge pull request #173 from tuncer/unknown-vsn

Fix false reporting of (plain) vsn strings
Merge pull request #174 from tuncer/dialyzer-fixes

rebar_core: fix Dialyzer warning introduced in aa46d85 (#157)