I'm also reaching out to ParrotSec about this.
They are using arachni-1.5.1 which is missing the arachni_web interface, so I "purged" that version after trying to get it to work with the same issues I have with the one I downloaded from Github.

Bundle install

Installing msgpack 0.7.0 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
creating Makefile

current directory: /tmp/bundler20181125-63989-1fogbv0msgpack-0.7.0/gems/msgpack-0.7.0/ext/msgpack
make "DESTDIR=" clean

current directory: /tmp/bundler20181125-63989-1fogbv0msgpack-0.7.0/gems/msgpack-0.7.0/ext/msgpack
make "DESTDIR="
compiling buffer.c
compiling buffer_class.c
compiling core_ext.c
core_ext.c: In function ‘MessagePack_core_ext_module_init’:
core_ext.c:135:22: error: ‘rb_cFixnum’ undeclared (first use in this function); did you mean
‘rb_isalnum’? rb_define_method(rb_cFixnum, "to_msgpack", Fixnum_to_msgpack, -1); ^~~~~~~~~~ rb_isalnum core_ext.c:135:22: note: each undeclared identifier is reported only once for each function it
appears in
core_ext.c:136:22: error: ‘rb_cBignum’ undeclared (first use in this function); did you mean
‘rb_isalnum’? rb_define_method(rb_cBignum, "to_msgpack", Bignum_to_msgpack, -1); ^~~~~~~~~~ rb_isalnum make: *** [Makefile:243: core_ext.o] Error 1

make failed, exit code 2

Gem files will remain installed in
/tmp/bundler20181125-63989-1fogbv0msgpack-0.7.0/gems/msgpack-0.7.0 for inspection. Results logged to
/tmp/bundler20181125-63989-1fogbv0msgpack-0.7.0/extensions/x86_64-linux/2.5.0/msgpack-0.7.0/gem_make.out

An error occurred while installing msgpack (0.7.0), and Bundler cannot continue.
Make sure that gem install msgpack -v '0.7.0' --source 'https://rubygems.org/' succeeds before
bundling.

In Gemfile:
arachni was resolved to 1.5.1, which depends on msgpack

This msgpack-0.7.0 appears to be outdated (latest is 1.2.4)
I tried to force using the 0.7.0 version, but it appears not to be available
I found it in the install, but cannot connect to it (maybe I don't know how).
If I force to use the latest versions - including some other gems, then the bundle install works, but arachni doesn't.

Any help here would be appreciated.
Thanks!