Add tests for formula spec selection

These tests document the relative precedence of the stable, bottle,
devel, and head specifications, and the conditions that can influence
which is selected (e.g. command-line flags).
Raise a useful exception for incomplete formulae
Make a specific assertion in this test
Remove obsolete tests

This behavior is now tested at more appropriate levels in
test_software_spec, test_formula_spec_selection, and
test_formula_validation.
Use assert_operator
Remove extra require
OpenSSL: enable ec_nistp_64_gcc_128 option on 64-bit machines

This is supposed to increase speed dramatically...

Closes #19064.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
spin: install ispin

Closes #19044.
rrdtool: use stdenv to support custom perl and ruby

Closes #19018.
New visualization for brew deps --tree

Closes #18835.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
New formula: libtar

libtar provides a C interface to tar files. It installs include and
library files as well as an executable called libtar, which provides
similar functionality to tar and gnu-tar. I think there should be
no conflicts since the executable has a different name.

Closes #18780.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
New formula: libusbx 1.0.14

Added homebrew formula for the popular user-space USB library, libusbx 1.0.14

Closes #17186.
Closes #18452.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
wireshark: modernize style
libusb: fix typo
cassandra 1.2.4

Closes #19187.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
vim 7.3.891

Closes #19188.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
libmatroska 1.4.0

Closes #19190.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
libebml 1.3.0

Closes #19189.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
lilypond devel 2.17.16

Closes #19191.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
Only prune taps if taps exist.

Closes #19192.