Merge pull request #489 from kojix2/table_style

fix table style

Author: Shekharrajak

lib/daru/iruby/templates/dataframe.html.erb

@@ -1,5 +1,5 @@
 <b> Daru::DataFrame<%= name ? ": #{name} " : ''%>(<%=nrows%>x<%=ncols%>) </b>
-<table>
+<table border="1" class="dataframe">
   <%= table_thead %>
   <%= table_tbody %>
 </table>