Add "libc6-dev" for the "pty" module

[tianon]

Before:

irb(main):001:0> require 'pty'
LoadError: Could not open library 'libutil' : libutil: cannot open shared object file: No such file or directory. Could not open library 'libutil.so' : libutil.so: cannot open shared object file: No such file or directory
    from /opt/jruby/lib/ruby/shared/ffi/library.rb:114:in `ffi_lib'
    from org/jruby/RubyArray.java:2412:in `map'
    from /opt/jruby/lib/ruby/shared/ffi/library.rb:84:in `ffi_lib'
    from /opt/jruby/lib/ruby/shared/pty.rb:10:in `LibUtil'
    from /opt/jruby/lib/ruby/shared/pty.rb:5:in `PTY'
    from /opt/jruby/lib/ruby/shared/pty.rb:3:in `(root)'
    from org/jruby/RubyKernel.java:1071:in `require'
    from /opt/jruby/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:1:in `(root)'
    from /opt/jruby/lib/ruby/shared/rubygems/core_ext/kernel_require.rb:54:in `require'
    from org/jruby/RubyKernel.java:1107:in `eval'
    from (irb):1:in `evaluate'
    from org/jruby/RubyKernel.java:1507:in `loop'
    from org/jruby/RubyKernel.java:1270:in `catch'
    from org/jruby/RubyKernel.java:1270:in `catch'
    from /opt/jruby/bin/irb:13:in `(root)'
irb(main):002:0> 

After:

irb(main):001:0> require 'pty'
=> true
irb(main):002:0> 

[cpuguy83]

LGTM