Fix for warnings on Ruby 2.5 in ERB binding

[aycabta]

ERB requires variable context as Binding object and Darkfish uses Kernel#binding for it, but it causes warnings "assigned but unused variable" on Ruby 1.9 or later, so there ware "suppress 1.9.3 warning" code like below:

# suppress 1.9.3 warning
variable = variable = expressions

The variable variable is assigned but unused at this scope and used inside ERB template, so the double assign of variable is for suppress the warning.

This code causes warning "assigned but unused variable" on Ruby 2.5 again. I fixed it with Binding#local_variable_set for "use" variable.

This Pull Request removes Ruby 2.5 warnings in tests below:

/path/to/rdoc/lib/rdoc/generator/darkfish.rb:317: warning: assigned but unused variable - asset_rel_prefix
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:347: warning: assigned but unused variable - asset_rel_prefix
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:348: warning: assigned but unused variable - svninfo
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:420: warning: assigned but unused variable - asset_rel_prefix
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:402: warning: assigned but unused variable - current
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:462: warning: assigned but unused variable - current
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:463: warning: assigned but unused variable - asset_rel_prefix
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:487: warning: assigned but unused variable - asset_rel_prefix
/path/to/rdoc/lib/rdoc/generator/darkfish.rb:544: warning: assigned but unused variable - asset_rel_prefix

[hsbt]

Looks good.