修正JiebaAnalyse::extractTags遇到關鍵詞是數字時的錯誤

[hippolin]

修正JiebaAnalyse::extractTags遇到關鍵詞是數字時，數字的關鍵詞會被清除，從0開始排。
例如下例文章中的2016會變成0：

台灣科技業向來以深厚的代工實力自豪，不過現在則是面臨產業轉型的問題。
今天有場2016年科技趨勢座談，討論的就是台灣IT產業如何面對未來。

extract tag:

    [產業] => 1.0692828963077
    [台灣] => 1.0692828963077
    [有場] => 0.53464144815385
    [問題] => 0.53464144815385
    [0] => 0.53464144815385
    [討論] => 0.53464144815385
    [未來] => 0.53464144815385
    [面對] => 0.53464144815385
    [IT] => 0.53464144815385
    [趨勢] => 0.53464144815385
    [轉型] => 0.53464144815385
    [面臨] => 0.53464144815385
    [業向] => 0.53464144815385
    [不過] => 0.53464144815385
    [實力] => 0.53464144815385
    [現在則] => 0.53464144815385
    [科技] => 0.44863651487769
    [代工] => 0.38121898482346
    [自豪] => 0.33183377282346
    [深厚] => 0.29984170844462

[codecov-io]

Current coverage is 92.94%

Merging #10 into master will not affect coverage as of 6972b30

@@            master     #10   diff @@
======================================
  Files            4       4       
  Stmts          723     723       
  Branches         0       0       
  Methods         25      25       
======================================
  Hit            672     672       
  Partial          0       0       
  Missed          51      51       

Review entire Coverage Diff as of 6972b30

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[fukuball]

Cool~ Thanks for your help!